On April 9, 1918, after a heavy bombardment in which over 60,000 gas shells were used, the Germans attacked British and Portuguese lines from La Bassée Canal to the neighborhood of Armentières (a distance of eleven miles).
The Portuguese in the center and the British on the flanks of the river Lys between Estaires and Bac St. Maur were forced to retire under the German pressure. Heavy fighting continued throughout the day in this sector. In the vicinity of Givenchy and Fleurbaix the British maintained their position and repulsed strong assaults. Richebourg St. Vaast and Laventie were occupied by the enemy.
In the morning of April 10, 1918, the Germans launched a new attack, with strong forces, against the British positions between the Lys River and Armentières and the Ypres-Commines Canal. The British were driven back to the line running through Wytschaete along the Messines Ridge to Ploegsteert.
South of Armentières, after a prolonged struggle, the Germans crossed the river Lys and established themselves on the left bank at a number of points east of Estaires and near Bac St. Maur. Southward from Estaires—the southern limit of the German offensive—the British continued to hold their lines. Givenchy was recaptured from the Germans, and 750 prisoners. The possession of this place was important to the Allies, as it stands on high ground, and is a gateway on the road to Béthune.
In the course of the two days' fighting in this region the Germans advanced in the center to a depth of about 5,500 yards, the British and Portuguese losing a considerable number of prisoners and guns.
In the retirement of the French forces to the line of the Ailette River two French battalions were cut off and the Germans took 2,000 prisoners. The retirement of the French was made on account of an awkward corner in the line.
The fighting during these days showed that it was the Germans' purpose to exhaust the British army. Their first plan had been to cut off the British from the French, but that failed. Now they attacked the British wherever they saw a favorable chance, hoping to destroy such large numbers that they would be unable to take the offensive.
The German advance between Ypres and Arras, March-June, 1918.